This invention relates to semiconductor charge transfer devices, and, more particularly, to virtual phase, buried channel charge coupled devices.
Charge transfer devices including charge coupled devices (CCDs) are well known monolithic semiconductor devices and are used in various applications such as shift registers, imagers, infrared detectors, memories, etc. For example, Wolfe and Zissis, Eds, The Infrared Handbook (1978), devote chapter 12 to CCDs and with emphasis on their use in infrared signal processing. The traditional three phase CCD is an array of adjacent cells (each cell, in essence, being a MIS capacitor) and operates by tying the gates of every third cell together so that varying (clocking) the gate voltages in three phases transfers charge packets from cell to adjacent cell. This traditional CCD would be basically formed by covering a p-type substrate with oxide and patterning metal gates on the oxide. In such a device electrons are transferred in the substrate essentially along the interface with the oxide, which leads to poor transfer efficiency due to traps at the interface. Further, such devices have problems including breakdown of the gate oxide, complicated gate connections, blooming (a charge packet larger than the capacity of a cell will overflow into adjacent cells), etc.
The problem of interface trap induced low transfer efficiency can be solved by using a buried channel CCD structure (BCCD) in which the charge packets are confined to and flow in a channel that lies in the substrate beneath the interface. Thus the charge packets are held away from the interface traps and do not exhibit low charge transfer efficiency. However, this arrangement increases dark current generation at the interface. A BCCD can be fabricated by forming an n-type layer on the p-type substrate and then covering it with an oxide layer and patterned metal gates on the oxide. The n-type layer is thin and is fully depleted by the gate clocking action and bias applied to an n+-type junction attached to the end of the channel. The conduction and valence energy band diagram for the device shows the bands that are bent and have a minimum in the n-type layer. This conduction band minimum is the location where the charge packets accumulates and are transferred from cell to cell by variations in the gate bias voltages changing the relative energy band minima between cells. For example, see, Sze, Physics of Semiconductor Devices (2d Ed 1980) pp 423-427.
A BCCD with the standard metal gate/oxide replaced by a p-type gate region to form a p-n junction with the n-type channel layer is also known; see, for example, E. Wolsheimer and M. Kleefstra, Experimental Results on Junction Charge-Coupled Devices, 29 IEEE Trans. Elec. Dev. 1930 (1982). In such a junction BCCD, the control of the energy bands in the buried channel is accomplished by varying the reverse bias on the p-n junction. But such a junction BCCD may have large leakage currents between the adjacent p-type gate regions and also has difficulty in achieving smooth transitions of the buried channel potential energy levels between cells.
The problems associated with the complicated gate structure of such three phase BCCDs are overcome with the virtual phase CCD as described in Hynecek, U.S. Pat. No. 3,2729,752. A virtual phase CCD needs only a single set of gates and a single clocking bias and operates on the principle of selectively doping different regions of each cell so that clocking the gate affects only the energy bands in a portion of each cell and, indeed, drives them from below to above the fixed energy bands in the remainder of each cell. The doped region that shields this remainder of a cell from the effect of the clock bias of the gate voltage is normally called the "virtual gate".
However, all of the prior art devices still suffer from problems of oxide breakdown, blooming, leakage currents, and nonsmooth cell-to-cell potential energy profile transitions.